New Zealand Aluminium Smelters bosses are focusing on keeping the Tiwai Pt smelter afloat as they decide whether to appeal an employment court decision that will cost them millions.

It is understood the review will include further cost reductions and continuous business improvement. It will also involve smelter bosses looking at how $20 million would be distributed to workers, including looking at options such as cash payouts or leaving the money on the books as a liability.

In May last year, 64 Engineering Printing and Manufacturing Union (EPMU) members won an Employment Relations Authority case they took against NZ Aluminium Smelters concerning lieu days.

The company appealed the decision and a two-day hearing was held in December. On Friday, the workers found out the appeal had been overturned.

The company said at the authority hearing that, if found liable, it would pay all affected staff, not just union members.

It is understood bosses initially thought the appeal decision could mean workers previously employed would have to be paid but now they were unsure.

General manager Gretta Stephens said the decision would have a "significant financial impact".

The company was passionate about providing long-term career opportunities for future generations of Southlanders and would work hard to make it a reality, Stephens said.

She would carefully review the employment court decision during the next few weeks to decide whether to appeal.

Until that process was completed, the company would be unable to answer questions regarding individual entitlements, she said.

"This decision is particularly disappointing as we are very proud of NZAS' record as a generous employer.

"The decision has come at a particularly challenging time for the smelter and will have a significant financial impact. However, we are continuing to do everything in our power to ensure the smelter's ongoing commercial competitiveness".

EPMU Southland organiser Trevor Hobbs said the union took the dispute on behalf of the 64 union members and it was smelter bosses who grossly increased the impact of the cost to the company.

Hobbs said he was recruiting workers to join the union at the time.

The instruction from company management was a deliberate attempt to stop workers from joining the union, he said.

The employment dispute centred on the interpretation of lieu days after workers at the plant changed from eight-hour shifts to 12-hour shifts in the 1990s.

The company continued to provide the 12-hour shift workers with an additional eight hours of paid leave in respect of each public holiday.